Accumulate intermediate results of a vector reduction

accumulate()reduces a vector with a binary function,
keeping all intermediate results, from the initial value to the
final reduced value, i.e. the result you'd have gotten if you used
reduce() instead of accumulate().

Arguments

.x

A list or atomic vector.

.f

For reduce(), and accumulate(), a 2-argument
function. The function will be passed the accumulated value as
the first argument and the "next" value as the second argument.

For reduce2() and accumulate2(), a 3-argument function. The
function will be passed the accumulated value as the first
argument, the next value of .x as the second argument, and the
next value of .y as the third argument.

...

Additional arguments passed on to the mapped function.

.init

If supplied, will be used as the first value to start
the accumulation, rather than using x[[1]]. This is useful if
you want to ensure that reduce returns a correct value when .x
is empty. If missing, and x is empty, will throw an error.

.dir

The direction of reduction as a string, one of
"forward" (the default) or "backward". See the section about
direction below.

.y

For reduce2() and accumulate2(), an additional
argument that is passed to .f. If init is not set, .y
should be 1 element shorter than .x.

Value

A vector the same length of .x with the same names as .x.

If .init is supplied, the length is extended by 1. If .x has
names, the initial value is given the name ".init", otherwise
the returned vector is kept unnamed.

If .dir is "forward" (the default), the first element is the
initial value (.init if supplied, or the first element of .x)
and the last element is the final reduced value. In case of a
right accumulation, this order is reversed.

Life cycle

accumulate_right() is soft-deprecated in favour of the .dir
argument as of rlang 0.3.0. Note that the algorithm has
slightly changed: the accumulated value is passed to the right
rather than the left, which is consistent with a right reduction.

See also

Examples

# With an associative operation, the final value is always the# same, no matter the direction. You'll find it in the last element# for a left accumulation, and in the first element for a right one:1:5%>%accumulate(`+`)

#> [1] 1 3 6 10 15

1:5%>%accumulate(`+`, .dir="backward")

#> [1] 15 14 12 9 5

# The final value is always equal to the equivalent reduction:1:5%>%reduce(`+`)

#> [1] 15

# It is easier to understand the details of the reduction with# `paste()`.accumulate(letters[1:5], paste, sep=".")

#> [1] "a" "a.b" "a.b.c" "a.b.c.d" "a.b.c.d.e"

# Note how the intermediary reduced values are passed to the left# with a left reduction, and to the right otherwise:accumulate(letters[1:5], paste, sep=".", .dir="backward")

#> [1] "a.b.c.d.e" "b.c.d.e" "c.d.e" "d.e" "e"

# `accumulate2()` is a version of `accumulate()` that works with# ternary functions and one additional vector:paste2<-function(x, y, sep=".") paste(x, y, sep=sep)
letters[1:4] %>%accumulate(paste2)